Anthropologically speaking, most women would marry and start a family in their teenage years. Physically, we are capable. That is why puberty begins between 8-18 years old, and peaks as a teenager. However, the way our society has adapted makes it nearly impossible to successfully start a family during these years.
The reason that it is so difficult to say "no" to sex as a teen is because we are made to start sharing our sexuality at that age. Since the legal age to get married became eighteen, our biological make-up has struggled to co-exist with today's society. In ancient times and in some cultures today, women would marry at a very early age, but our culture has raised the bar. Now our education, which takes up the vast majority of young lives' time, causes us to practice abstinence which can be difficult for those without developed self-control, considering that we do not offer class called "Self-Control 101".
I say, "Don't blame the teens, blame society!" Firstly, if adults don't expect teenagers to be able to make important decisions in their lives, then who will? Chances are that there hormones will. If young adults are not being taught vital moral and life lessons at a young age, they might never be exposed to them. Sex is a big issue in today's society, and I am still trying to conjure up a solution. If we want the teenage STD epidemic and pregnancy issue to go away, we are going to have to do a lot more than telling them that it's the "right" thing to do. Simply telling them to stop will not do anything. They know that its "wrong" in our opinion, and that is why they do it.
Sex is not that big of a deal to teenagers in this day in age. However, sex is not necessarily an act of passion, but an act of rebellion to some of them. Adults tell them not to, and because adolescence is a time to take risks and find yourself, "why not?" We cannot blame the generation for their "wrong-doing" because we have basically set them up for catastrophy. The Bible is telling them to wait until marriage, but when the Bible was written, marriage occurred at a very young age (13-16 years old). Do you see a trend? Big changes need to be made in order to fix this because we cannot blame the adolescents.